Reputation on the line with new 6MW

WORLDWIDE: The 6MW successor to Siemens' 3.6MW offshore workhorse, the SWT-6.0-120, is being tested and is almost ready to come off Siemens' production lines.

The new Siemens SWT-6.0-120

The firm expects the first project with the new turbines to be built by 2013. Eize de Vries takes a look at its development.

From a distance, the huge structure with its distinct nacelle, red helicopter hoisting platform and 120-metre rotor could be clearly distinguished among several other small test turbines.

It is an impressive view, and so it should. This is arguably the most prestigious product that will soon be coming off Siemens production lines. The German-based company is the world leader in offshore wind with its 3.6MW turbine, considered the offshore market's workhorse. The 6MW direct-drive SWT-6.0-120 offshore turbine prototype is its successor and the firm's reputation rests on its success.

The prototype, which is being put through its paces at the Riso technical university test site in Hovsore, Denmark, has been put on the same concrete foundation base built eight years ago for the prototype of the 3.6MW Bonus (Siemens purchased Bonus Energy in late 2004). When I visited that turbine in 2004, what caught my attention was the foundation design containing two cast-in tower-mounting bolt circles with the 3.6MW tower already bolted onto the inner circle.

The outer ring was to accommodate a future 5MW turbine, part of the design strategy of chief technical officer Henrik Stiesdal, who was then working for Bonus. Each new model would be developed with a higher power rating and larger rotor diameter.

The model would be upgraded and optimised later by increasing power rating and rotor size while largely retaining head mass (the weight of the nacelle and rotor). Such a strategy sought to lower development risk by compensating for initial gaps in technical know-how.

This successful optimising strategy came to an end after the geared 2.3MW and 3.6MW turbines were introduced in 2002 and 2004 respectively. The smaller 2.3MW turbine, which targets the volume market has maintained the power rating unchanged until today, but rotor diameters have increased in four steps - from 82 metres, to 93 metres to 101 metres and to 108 metres.

The 5MW turbine was never built. When Siemens bought Bonus it instead introduced an upgraded 3.6MW sister model with its rotor enlarged from 107 metres to 120 metres in 2009.

Thin blades

The company's main aim was to boost yield with only minor increases in turbine loads. It achieved this through the first in a new product family, the slender 59.5-metre B58 rotor blade.

This B58 blade and SWT-3.6-120 rotor hub are also found on the SWT-6.0-120, but in late 2011 a larger 75-metre B75 blade was introduced on the market. The resulting new SWT-6.0-154 has a record 154-metre rotor diameter that boosts rotor swept area by 65%.

The turbine is expected to generate 20-24% extra energy compared to the SWT-6.0-120 at 9-10 metres per second (m/s) average wind speeds. The B75 is again built in glass-fibre reinforced epoxy composite without using carbon, an expensive material also known to enhance manufacturing complexity. Despite leaving out carbon, blade mass could be limited to only around 25 tonnes.

Siemens developed two variations of its B52 blade at the same time for the 3.6MW turbine - one in segments, the other a single piece. The company aimed to find out if segmented blades could resolve possible problems with road transport for long blades. However, after reviewing the options it decided to continue with a single-piece B52 blade. Segmentation was not even considered for the subsequent B58 and B75 blades.

Inside the 6.0

The new SWT-6.0-120 is fitted with an elevator, and only the last stretch into the nacelle is climbed by ladder. The central entrance area is spacious and partially confined by the huge cast nacelle bedplate's outer shape.

Towards the rear of the nacelle, is direct easy access to two parallel-mounted power electronic converters while the surrounding floor space is largely flat. The spacious area makes life easier for service personnel during their work in the nacelle and in cases where they might have to spend the night there due to bad weather. There is even space for a workbench and storage for tools.

The generator winding is electrically split into two halves. Each section operates as a separate electrical machine that feeds current through an individual converter. This built-in redundancy enables continued operation at reduced capacity - maximum 50% - if a generator or converter section develops a failure.

The medium-voltage transformer is located in a fully enclosed explosion-protected reinforced area under the converter cabinets, and a single 33kV AC-cable feeds power down the tower. Walking in the opposite direction through the hollow main shaft there is a single rotor-bearing inner ring and access to the rotor hub. For safety reasons, direct hub access is blocked by a mesh-wire steel door, which can only be opened once the rotor lock is engaged by a hydraulic locking pin.

All current Siemens turbine models operate with hydraulic rotor-blade pitch as standard control technology, enabled by two hydraulic cylinders and accumulator energy storage per individual blade. Each of these blades has a built-in capability to function as an independent fail-safe aerodynamic brake that enables the turbine to come to a safe stop.

Big bearing

The prototype is fitted with a rotor bearing with an outer diameter of around 3.5 metres. This will be increased to 4 metres with the enlarged 154-metre rotor, making it one of the largest rotor bearings currently manufactured in series.

The ring generator has a 6.5-metre outside diameter and is built around the hollow shaft. Rotor enlargement affects the dimensions of the cast main carrier and hollow shaft diameter, and ensures that there is still sufficient space for accessing generator-cooling fans located around the generator. These fans blow cold air through the gap between stator and generator rotor.

Another interesting design feature is that hot air from the generator is centrally collected and then fed through a passive water-cooler type heat exchanger without moving parts being integrated into the nacelle platform. In the future, to further reduce complexity, converter cooling might be shifted to the nacelle cooler, where there is some spare capacity.

Both generator diameter and generator length will be retained with the SWT-6.0-154. This is despite the fact that rotor speed has to be reduced because of the enlarged rotor diameter, which increases torque level and internal heat production with unchanged power rating. Stiesdal says this has been made possible due to substantial generator thermal reserves. He also indicated that as an additional measure, more generator cooling fans will be incorporated to increase heat dissipation.

Siemens has a strategy to standardise component use as much as possible. The SWT-6.0-120 is fitted with the same yaw motor drives as for the 3MW SWT-3.0-101 direct-drive turbines, but the number has increased from eight to 16.

The first B75 rotor blade was produced in January this year and an SWT-6.0-154 prototype is planned for this summer. "The 6MW turbine technology conceptually builds on the SWT-3.0-101, now a series product," Stiesdal concludes. "Our next aim is to build a first project with our 6MW turbines in 2013."

THE COMPETITION OFFSHORE RIVALS

Although Siemens dominates the offshore wind market with its 3.6MW workhorse model, it is expected to face much stronger future competition from an increasing number of international firms:

In the 5-6MW+ class, only a few suppliers offer commercial products and have a varying track record with offshore experience. These include in particular the Areva M5000-116, Bard 5.0, and Repower 5M/6M sister models. New 5-6MW+ prototypes include the Alstom Haliade 150 6MW (installed in 2012), Bard 6.5 (2011), Sinovel SL5000 (2011), and XEMC XD115 (2011).

Different 5-8MW offshore turbines in various stages of development include the DSME 7MW, Gamesa G128-5.0MW and G11X-7.0MW, Vestas V164-7.0MW, NPS 8.0-175, and the 10MW AMSC Windtec SeaTitan.

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